Optical heads for imaging a plurality of light spots on a light sensitive medium such as printing plates often incorporate, as a light source, an array of pigtailed laser diodes. Each laser diode is optically coupled to a proximal tip of a multimode optical fiber. The distal ends of the optical fibers are supported in a linear array by mechanical means such as V-groove plates.
FIG. 1 is a schematic representation of such a prior art imaging system comprising a light source 10, e.g. laser and coupling optics 20 coupled to a multimode optical fiber 30 having a circular core shape. The distal end of multimode optical fiber 30 is supported by mechanical support 40, such as micro-machined V-grooves, and emits light through a telecentric lens 50 onto an imaging surface 60. Imaging surface 60 may be a photosensitive printing plate, including a thermally ablative printing plate.
An important characteristic of any fiber-coupled laser diode is the general distribution of light energy exiting from a tip face at a distal end of the multimode optical fiber. Preferably, this distribution is relatively even or homogenous. The homogeneity of the distribution depends on a variety of parameters such as the quality of the supplied laser light which can for example depend upon way the laser diode is modulated, the optical characteristics of the optical coupling between the laser diode and the optical fiber, the length of the optical fiber, the bending along the fiber, etc. which may cause non-uniform and time-dependent energy distribution of the light emerging from the distal end of the multimode optical fiber. This often leads to unpredictable energy distribution in the writing spot and to undesired effects on the image. It is well known that the radiation at the output of a regular circular multimode fiber is not steep, not homogeneous and has many modes. Furthermore, the mode structure of this radiation is dependent on the fiber lay and changes when the fiber is moved, as happens in many imaging machines.
Several ways of avoiding this effect are mentioned in the prior art. For example, EP 0992343 A1 to Presstek Inc. uses a controlled-angle diffuser. The diffuser introduces scrambling in the angular energy distribution and thus smoothes it. This approach, however, cannot correct asymmetrical spatial energy distributions, such as doughnut-mode energy distributions.
Another way is to use a non-circular hollow waveguides as described, for example, in the article entitled “Beam Homogenizer for Hollow-Fiber Delivery System of Excimer Laser Light”, published in Applied Optics, volume 42, no. 18, Jun. 20, 2003. A hollow waveguide is usually made from silicon coated with a reflecting metal layer. However, hollow wave-guides which use the principle of reflection, absorb part of the guided light converting it to heat. This wastes energy and can negatively influence the shape of the wave-guide or the direction of the wave-guide.
U.S. Pat. No. 6,519,387 to Sunagawa et al. claims the use of an elongated core shape at the distal end of a fiber to improve beam characteristics. However, the use of non-circular cores at the distal ends of the fibers causes the spot configuration of the scanned beam to be short in one direction rather than circular. Further, for example, it is harder to align and orient the fibers in mechanical supports such as V-grooves.
Thus, there is a need in the printing arts and in other fields for a system that provides homogenized steep profile of radiation from a fiber which has a circular core. This may improve the image quality of printing systems.